1. Field of the Invention
This invention pertains generally to preparing inorganic mesostructured materials, and more particularly to preparing patterned inorganic mesostructured materials, with controllable orientational ordering for anisotropic structural, mechanical, optical, reaction or transport properties for applications in opto-electronics, separations, fuel cells, catalysis, or MEMS/microfluidics.
2. Description of Related Art
A great deal of recent research towards obtaining orientational ordering in mesostructured inorganic materials has focused on producing materials with non-linear optical properties. Therefore, this brief review of the state-of-the-art will focus on the type and performance of different optical materials. Much research has centered on the alignment of optically-responsive guest species within a host matrix. While a wide range of guest species have been used, host matrices have focused on polymer or inorganic materials. Work on polymer systems has included dissolving optically-responsive molecules within the polymer, incorporating the optically-responsive species as covalently bonded side chains on the polymer backbone, or designing the polymer molecules with the guest species incorporated directly into the backbone chain of the polymer. In addition to polymer host matrices, inorganic glasses composed of oxides of silicon, aluminum, or titanium have been synthesized through sol-gel processing with optically-responsive guest species incorporated. Finally, in an attempt to combine the beneficial properties of both polymer and inorganic host materials, sol-gel derived hybrid organic-inorganic materials have been investigated for optical applications.
Inorganic single crystals, such as lithium niobate or potassium dihydrogen phosphate, are currently the most widely used materials in electro-optic devices. Major drawbacks of these single crystals include the high pressures and temperatures required during materials processing, large materials cost, and poor environmental stability. To overcome the obstacles presented by inorganic single crystals, guest-host systems using polymer or inorganic glass host matrices containing homogeneously dispersed, non-linear optically-responsive species have been developed. In both types of host materials, the guest species are aligned through the application of an electric field. Electric field poled systems of polymers such PMMA doped with the chromophore 4-(dicyanovinyl)-4-(dialkylamino)azobenzene have shown chromophore densities up to 2.3×1020 cm−3 and poling alignment of d33=74 pm/V. However, the poling alignment of the sample had decayed to d33=19 pm/V after several days at room temperature. This example illustrates both the possibility for high guest molecule solubility and the poor thermal stability of polymer host materials. In contrast to polymer systems, sol-gel-processed wholly inorganic or organically-modified inorganic glasses exhibit greater thermal stability, but lower guest molecule solubility. For example, poling alignment of d33=60-100 pm/V and temporal stability of up to 10,000 hours have been reported for silica-organic systems containing functionalized azo dyes, such as Dispersed Red. Unfortunately, these organically-modified inorganic glasses only allow guest molecule doping concentrations of up to ˜5 wt %.
Transparent mesostructured inorganic-organic hybrid materials with aligned mesopores provide a means to combine the beneficial properties of both polymer and inorganic glass host materials. Self-assembled surfactant-templated mesostructured materials allow for tunable guest-host interactions due to the numerous periodic structures (e.g., lamellar, 2D or 3D hexagonal, cubic, worm-like), pore dimensions (3-50 nm), structure-directing agent (SDA), guest species, and framework compositions possible. In addition, the sol-gel processing involved in the ‘bottom-up’ synthesis of these materials allows for convenient, inexpensive device fabrication. However, conventional mesostructured inorganic films generated by solvent evaporation using spin-coating or dip-coating generally have micron-size domains that are randomly oriented over macroscopic length scales. If mesostructured inorganic-organic materials are to be used in non-linear optical or other anisotropic membrane applications, materials with aligned mesochannels need to be attained over macroscopic length scales.
Several techniques have obtained limited success in gaining precise control over mesopore alignment. Magnetic field alignment has been used to synthesize hexagonal mesostructured silica, but radial integration over the width of the (100) reflection in the 2D X-ray diffraction pattern indicates only partial alignment with a distribution of domain orientations of 53° FWHM. An alternative method to obtaining mesochannel alignment involves treating the substrate with a rubbed polymer prior to deposition of the mesostructured inorganic-organic film. Mesoporous silica deposited on PTFE-rubbed glass substrates showed a distribution of domain orientations of ˜15° FWHM. More recently, polyimide-rubbed substrates have produced degrees of alignment up to ˜30° FWHM. While the degree of alignment has been significantly improved over the last decade, these techniques only produce mesochannel alignment parallel to the substrate. Researchers at IBM have formed continuous non-patterned silica films with mesochannel alignment perpendicular to the silicon substrate by treating the sample in a chamber of chloroform and octane vapors, which is difficult to control.
Incorporation of small molecule chromophores and semiconducting polymers into aligned hexagonal mesoporous silica films has demonstrated non-linear optical properties. Highly polarized luminescence was acquired from poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] imbibed into mesoporous silica films produced using the polyimide-rubbing technique. In addition, low-molecular weight cyanine dyes doped into polyimide-rubbed mesostructured silica films have exhibited polarized visible light absorption. These reports illustrate the anisotropy of guest species inside aligned mesochannels of mesostructured inorganic-organic hybrid films.
Mesostructured inorganic-organic hybrid materials have been formed into films, fibers, monoliths, and micro-patterns made by soft lithography. For optical device applications, thin films or monoliths are the preferred morphologies. Typically, during the synthesis of thin films, a sol-gel solution containing a structure-directing agent, soluble metal oxide precursors, water, and diluted in a volatile co-solvent such as ethanol, is either dip-coated or spin-coated onto a substrate. Subsequent patterning of these films has traditionally been performed by conventional photolithography or self-assembling ink-jet printing. The mesostructures of thin films made in this fashion are characterized by domains consisting of nanoscopic structures that are well ordered over areas of ˜1 μm, but these micron size domains are randomly oriented over macroscopic length scales.